I'll leave it to others to actually hazard answers to your question, but I think the 1930 incident that you refer to is sufficiently curious to be worthy of some attention in its own right.

It does seem to be true that the announcement of "No news today" was made; the BBC's own Yearbook for 1930 makes mention of the broadcast. But it may help to contextualise. The announcement came at a very early point in British news broadcasting history. The BBC had only been permitted to prepare its own news bulletins, rather than broadcasting already-prepared copy produced by a press agency, in 1928; in 1930, the broadcasting of news was still the responsibility of the Department of Talks, and no separate News Section would be established until 1934. There were no portable sound recorders and no easy means of sending broadcast-quality sound back to a central studio from the field; it was only in 1936 (on the occasion of the great fire that destroyed the Crystal Palace in south London) that a telephone report, with the sounds of shouts, fire engines and flames in the background) was first broadcast live.

Furthermore, 18 April 1930 was Good Friday – a then fairly strictly observed public holiday on which British newspapers did not publish. This latter circumstance created a significant issue for the BBC, since at this point in its history it did not have its own journalistic staff. The existing news organisations of the day had bitterly opposed any suggestion that radio be allowed to become a real competitor to the press, and it was prepared to insist on the copyright it held on its own bulletins to prevent their being used as a source by the BBC.

The Corporation - which was and is publicly funded via payment by its audience of a licence fee – was thus forced to choose between developing its own, enormously expensive, news gathering organisation from scratch, something the newspapers would have decried as a waste of public money, and of reaching an agreement with the printed press. News gathering was not then seen as central to the BBC's mission - even though this was famously defined by its first Director General, John Reith, as "to inform, educate, and entertain" – so, under a 1924 agreement made with the two main wire news services of the day, the Press Association and Exchange and Telegraph, it maintained a staff of two editors and two sub-editors to go through the agency tickers to prepare news bulletins. The BBC made no claim to creating or even curating news; a surviving recording dating to 1936 reveals that a copyright notice stating that the news was "Copyright by Reuter, Press Association, Exchange Telegraph and Central News" was read out before the broadcast itself began.

All in all, then, the radio news of 1930 was, according to its historian Tim Crook, "an unselfconsciously amateur operation" which was "held in contempt by Fleet Street journalists" - that is, the staffs of the country's intensely competitive national newspapers.

It would be very interesting to know in more detail than we now do exactly what combination of circumstances led to the announcement you cite. The newswires themselves certainly did continue to operate over holiday periods, but it's possible at least some of the BBC news editing team were on holiday on the Good Friday in question, just as at least some of their newspaper colleagues - with no Good Friday papers to produce - would have been. I would suggest it's also extremely possible that the BBC team had by this point developed the habit of turning to the press of the day for guidance as to what news stories were considered most important and pressing, and using these leads to arrange the radio news bulletin. (It would be fascinating to run a study of any surviving news broadcasts of scripts against the same day's London evening newspapers to check on this, but certainly it does not seem impossible that the BBC's shoestring news operation lacked the experience and confidence to shape a news agenda on its own, and that its "unselfconsciously amateur" ethos did the rest.) Certainly I don't think that anyone working for the BBC at this point in its history considered that it was the Corporation's job to do more than act as a digest of already available news produced by other sources. There were literally dozens of newspapers, from The Times downwards, selling many millions of copies daily, that were already doing that job.

One further point worth making – which is a factor stressed by the BBC itself these days – is that in 1930,

those in charge of the Talks Department, where News was based, drew a definite distinction between "BBC news values" and "journalistic news values".

It was an absolute rule there should be no "sensationalism". Parliamentary news, not known for its ability to grip the listener, was given special prominence.

Not surprisingly, parliament had not sat that Good Friday, and so it was impossible for the staff on duty that evening to rely on one of the most usual "leads" for the nightly news bulletin.

The reality, then, was that the 18 April announcement was largely a product of the absence within the BBC of any team capable of generating its own news agenda in the absence of the usual guidance provided by the proceedings in parliament and by the newspapers of the day. Even as late as 1936 - at a time of growing international crisis, let's not forget - a junior BBC employee called Richard Dimbleby (later to become an extremely eminent broadcaster in his own right) could still write to the Chief News Editor that

a member or members of your staff – they could be called 'BBC reporters, or BBC correspondents' – should be held in readiness, just as they are at the evening paper men, to cover unexpected news for the day. In the event of a big fire, strike, civil commotion, railway accident, pit accident, or any other major catastrophe in which the public, I fear, is deeply interested, a reporter could be sent from Broadcasting House to cover the event for the bulletin.

At the scene, it would be his job, in addition to writing his own account of the event, to secure an eyewitness [and Dimbleby went on to give an earnest definition of how an "eyewitness" was to be defined]... and to give a short eyewitness account of the part he or she played that day. In this way, I really believe that News could be presented in a gripping manner.

Furthermore, the BBC broadcasting schedule for 18 April 1930 reveals that there had already been broadcasts of national and regional sports bulletins earlier that same evening. These seem to have gone ahead as normal; it was only the 15-minute political news broadcast scheduled for 8.45pm that was affected.

The idea that anybody thought there was literally "no news" on 18 April 1930 is thus pretty implausible. The phrase may have been the product of an incautious or hurried scriptwriter, and I suspect that whoever was responsible would have been pretty amazed to see their phrase entering the historical record, and being discussed so earnestly here on AskHistorians 87 years later.

Sources

Tim Crook, International Radio Journalism: History, Theory and Practice (1997)

Jonathan Dimbleby, Richard Dimbleby (1975)

Jackie Harrison, News (2005)